My explanation: [interesting aspects of the book]
1) Lisbeth Salander is a fascinating character. I'd love to live inside her mind just for a little while... Also, I've heard that the next book in the Millenium series, "The Girl Who Played With Fire", focuses much more on her, which is great news.
2) Martin Vanger is truly a shocking psycopath. The stuff he did was just disgusting and sick. When he talked, I could see his insanity so clearly it scared me to death. All those women he raped and murdered, including his sister Harriet, deserved to be properly avenged, instead of forgotten. And the poor cat he disemboweled and burnt... I can't even think about it.
3) Gottfried Vanger was a terrible father. He raped his own son and daughter, for God's sake! His son, Martin, was initiated in his father's murders at a young age. When Gottfried died of drowning, Martin took his place and continued to kill women and rape his sister. Gottfried's serial murders involved freaky Bible quotes and were simply barbaric. I wouldn't be able to describe them properly... You'll have to buy the book and read about all of them.
4) Mikael Blomkvist is a pig. "Mr Ladies Man" and Erika are very happy, blah, blah, blah. Lisbeth goes back to her depression because she's in love with him. For the first time she opened her heart to a person, she gets hurt. It's not fair; she's an amazing woman and doesn't deserve to feel like a freak.
The Big Conclusion:
It's an amazing book, one of the best I've ever read. Don't let yourself be intimidated by the terrible crimes featured in it: you can always stop reading if you feel you won't be able to stand the descriptions, though in my opinion a little vomit shouldn't stop you from reading such an excellent book. That's why I recommend it very strongly.